Carl's character folder in the Hall of Records on The Simpsons website.

Carl is an African-Icelandic-American, and a Buddhist with a master's degree in nuclear physics, fond of bowling and drinking at Moe's Tavern. Carl demonstrated his talent for science[1] and is frequently said to be among the most attractive men in Springfield.[2] It is suggested that he might be diabetic.[3]

Carl often serves as a taciturn straight man to Homer and company, probably because of his harsh opinions of others. On one occasion, he makes everyone in Moe's Tavern cry and then turns to the camera, explaining: "See, this is why I don't talk much.[4]"

There is a constant allusion to homo-eroticism in Lenny's and Carl's relationship, although no one seems to care much about it. Lenny dreamily recalled carving "Mount Carlmore", a huge bust of Carl in the side of a hill, during "one wonderful summer". When Homer asks Lenny what Carl thought of it, Lenny replies "You know, we've never discussed it.[5]"
When various characters looked at the stars to "see into their souls" during a blackout in Springfield, Lenny sees an image of Carl, and Carl sees an image of himself.[6] On another occasion, Lenny and Carl are at the Springfield Baseball Stadium, watching the Kiss Cam, and Lenny says: "Hey Carl, remember when we used to kiss like that...[pause] with our respective girlfriends?" Lenny once said he can't tell apart the remains of his sculpture with Carl's remarking "I don't know where Carl ends and I begin!", Carl then says "it's stuff like this that makes everyone think we're gay.[7]" Implying that they view themselves as being of the straight orientation. (His and Lenny's straight orientation is also implied in The City of New York vs. Homer Simpson when he and Lenny drunkenly suggest to Barney that he take them to the Playboy Mansion and the Girls College, respectively.)
At times, Carl gets annoyed at Lenny. When Carl learns that he is paired with Lenny in a team-building exercise on a power plant retreat, his irritated response is, "Aw, nuts. I mean... aw, nuts.[8]" Carl, along with Lenny share a similar styled surname: Carlson and Leonard. At one point, Lenny mentioned, when a fire was about to hit the flammable district during Springfield's massive fire, that their mother was working there.

In the Simpsons Game, Carl and Lenny are shown arguing while bird-watching, and having forgotten Lisa and Bart's names when they save Lenny and Carl from logging machinery, even though Lenny and Carl have known them all their lives. During a conversation in the Springfield Hub, Carl and Bernice Hibbert are hinted to have had a past relationship, that Carl wants to rekindle. He also mentions in this game his support for Obama.

In the early seasons, Carl was rarely seen with Lenny and did not have a consistent voice - on some occasions, he can be heard with Lenny's voice and vice versa. In an early 1991 episode, "Principal Charming", Carl's name is spelled "Karl".